Tag Archives: vertebra

Our new Fossil Hall will feature a trio of fin-backed predators from 285 million years ago, dug from the famous Red Beds of north central Texas. (If you would like to learn more about our expansion and how you can donate to the HMNS’s second century of science, click here.)

One of our Dimetrodons is a monster – as big as they get – with a live weight as big as a Siberian Tiger. That’s 500 pounds.

We’ve got parts from the head end, the rump, the shoulders. But up till now we didn’t have a perfect head-neck swivel joint, known technically as the atlas-axis complex. This is where the skull meets the neck vertebrae, and it’s the most complicated anatomical unit in the entire backbone.

Just a few week ago we scored the entire swivel apparatus. The bones come from a brand-new site where a single Dimetrodon was buried by a spring flood. The bones are beautiful. Each vertebra is complete and the rock is so soft it can be removed with the judicious use of a fingernail.

Head-Neck Muscles

The head-neck bones tell us secrets about how Dimetrodon acted in life. Here’s a basic diagram for a giant D’don, taken from the superb skeleton at the Smithsonian. The head-neck swivel complex in red. Check out the big prong sticking straight up – that’s the neural spine of the axis. And here’s a close-up of the bones. The neural spine of the axis is the biggest component.

There’s a thick muscle attached to the neural spine that runs forward to connect to the back of the skull.

Reach around and touch the back of your own neck – you’ll feel the muscle. Technical name: rectus capitis posterior (meaning: “pulls the neck straight back”). The great height of the neural spine in our D’don means an exceptionally powerful action in pulling the head back and up. That would be useful when grabbing big, struggling prey.

There is a second set of thick muscles that’s attached to the neural spine and runs outward to the outer corner of the back of the skull. Technical label: obliquus capitis. Meaning: “Muscle that pulls the head obliquely.” This muscle turns the head sideways – also useful when wresting with prey.

Joint Mechanics

Now let’s investigate how the joints operate. The first vertebra behind the head is the atlas. The name comes from a fancied resemblance to the Greek hero, Atlas, who held the globe in his muscular arms. The atlas vertebra has two parts, one above the other, that together make a cup. Into the cup fits a ball on the rear of the skull.

Check out the cup in our specimen. We’ll exhibit the neck bones in a special case, so visitors can get a sense of how the living D’don was an active, dynamic bio-machine.

D’don is very close to the direct ancestry of warm-blooded, hairy mammals, including us humans (we are all warm-blooded, though some of us are hairier than others). We will exhibit a cast of a human neck joint next to the D’don to demonstrate how much evolution has taken lace since Red Beds times, nearly 300 million years ago.

If you enjoyed my post on Dimetrodon bones, be sure to check out my recent posts on the skull of the Ceratosaurus and Archosaurus.

For the past month or so, it has felt very much like Christmas in the cave down in the basement (my nickname for our small but homely Paleontology lab). Recently, we opened a freshly recovered plaster jacket containing the arms of a great beast, the Permian finback Dimetrodon, and hope to exhume his ancient limbs soon. However, there have been several positive delays that have hindered our progress. This jacket has become the gift that keeps on giving, and has yielded much more than was expected.

After retrieving the jacket (click here for an example of this process) and carefully re-depositing it in our lab for meticulous preparation, we began digging from the top of the jacket – however, the Dimetrodon’s arms that we discovered on the surface are at the bottom of our jacket due to the fact that we always flip the jacket over in the field in order to extract it. Thus, we have a certain amount of sediment we must carefully burrow through, much like a blind mole, before we get to our objective – the arms. As a result of this process, anything in between is an unexpected, though happily accepted, added bonus.

The oddly-shaped Diplocaulus.

Our first bonus from the plaster casket was an immediate discovery and occurred during the “jacket flipping” process. This was a very fortuitous event, as we would later discover. After the 300 lb jacket had been flipped, one of our veteran diggers noticed the hint of a small, square bone just barely revealing itself from the moist red clay lining the surface of our upside-down jacket. The identity of the fingerprint pattern on the bone was undeniable: the boomerang-headed amphibian Diplocaulus. 290 million years in the dark red beds of what is now North Texas only to be tossed from its deep sleep in a matter of seconds. Our hearts raced at the thought of a complete Diplocaulus skeleton, as we had but bits and pieces of the odd creature to date. And what an odd creature it was. Imagine a three foot salamander with a boomerang for a head.

Notice the unique texture on the fossil –
a sure sign you’ve found
a Diplocaulus fossil.

Ignoring the desire to uncover the bone further, we shrouded the exposed surface with layers of aluminum foil and entombed the new specimen once more in plaster for the long haul to Houston. We now had two specimens that would need scrupulous attention upon reaching the lab.

The jacket was finally opened about a month later. Our newest paleo volunteer, Meredith, was assigned to work with me on the jacket. I opened the aluminum foil and plaster lid, re-exposing the red clay that I knew housed the Dimetrodon limb some three hundred millimeters or so below the surface. The small piece of Diplocaulus skull peered up at us. It was beautiful. The fingerprint pattern of the skull was astonishingly clear and well preserved.

Volunteer Meredith Fontana
holds the maxilla, the facial
skull bone that contains most
of the teeth of a large
Dimetrodon

The prep process began under Meredith’s careful fingers. Our speed of prep was severely diminished due to the presence of our newest Diplocaulus – and hopes that it would reveal more than a fragment of skull. Soon more skull fragments appeared. And more. Then ribs. Then vertebrae. All so very tiny. The largest vertebra was a mere seventeen millimeters. The entire animal in life was no larger than a cat. The ribs are quite peculiar; very flat and uncurved. Imagine this curious creature with a remarkably flat belly that clings advantageously to the bottom of a mud-filled wallow or other small body of water.

The eruption of bone continued, and is continuing. The possibility of having our very first complete Diplocaulus, or boomer-head as we call them, is a very distinct possibility. I realized this animal did not yet have a name. All our Permian pets receive names, mind you. After discussing the possibilities with our well-seasoned digger Johnny “The Mole Man” Castillo, we agreed that “Meredith” would be the name of our little amphibian. After all, our volunteer Meredith had been the sole prep-tech for this jacket.

Johnny Castillo prepares Meredith for
removal by adding a mini plaster jacket to
the skeleton to keep each bone in place.

The next bonus appeared a week ago; a beautiful, small, jaw full of teeth. Twenty of them, to be exact. Upon first glance at this handsome little jaw, I assumed I was seeing more of our dear Meredith, though something seemed rather odd. The teeth were not as needle-like as they should have been to be Diplocaulus; the jaw was not as round. I noticed that one of the teeth was loose. This was both good and bad. Re-attaching teeth to jaws is exceedingly complicated, and when you have a jaw that is only six-and-a-half centimeters long with itty-bitty teeth… We like to be perfectionists when it comes to prep and pre-dino dentistry is quite difficult.

This loose tooth was my chance to attempt to i.d. its owner. Under a microscope I stared at this magnificent crown in the palm of my hand. My heart stopped. Tiny serrations lined the outer edge. This wasn’t Diplocaulus. Who had serrations at this time? Dimetrodon did. But was this Dimetrodon?

Extremely tightly packed teeth may offer
an idea on how young Dimetrodons
regrow their teeth.

I looked at the tiny jaw. All the teeth were the same size. Except for a missing tooth at the front of the jaw which would have been obviously larger, but not by much. My heart skipped a beat this time. It was strikingly similar to a specimen possibly new to science discovered right here in our very own labs. It was only just recently we discovered a jaw which may soon prove to be a new genus of Sphenacodont. Sphenacodonts, a family of Pelycosaurs which includes our favorite fin-back Dimetrodon, were the first animals to evolve a specialized set of teeth which include a large canine tooth as well as smaller cutting teeth. Thus they are less like reptiles and more like mammals. Our distant cousins, these mammal-like-reptiles, would probably have been endothermic, or warm-blooded, as well.

I compared the two jaws closely. Both jaws contained the two sizes of teeth; the larger canine and the smaller canines, but the position of the teeth was a bit different.

I stared in disbelief; my heart had resumed beating as I thought about the other possibilities. Loomisi was another Dimetrodon species we were familiar with who also had serrated, tightly-packed teeth, and as Dr. Bakker suggested to me, a young Loomisi may be similar to the new specimens. Unfortunately, part of the jaw is missing. Consequentially much more analysis will have to be done before a final identification can be given.

What could this be?

The arms of the great Dimetrodon are still buried, all thanks to an amphibian named Meredith and some other strange beast who got in the way. The arms will have to wait while Christmas in the cave continues. The dust from the white plaster jackets fall to the floor like a fine powder snow, and our plaster gift keeps on giving. A Permian present wrapped in plaster and burlap, filled with bizarre creatures most of us only dream about; amazing creatures that lived 290 million years ago that tell us a story of what our planet was like before the first dinosaurs.